In most modern gas turbine engines, the compressor and turbine sections each includes at least one stage of blades which are mechanically attached to the rim of a disk which rotates at high speeds. However, engine designs which incorporate mechanical blade attachment schemes suffer from several inefficiencies. Most notably, during engine operation, air leaks through the attachment area, and such air is therefore not available to provide compressive thrust to the engine. Furthermore, mechanical blade attachments configurations add additional weight to the disk, which is undesirable. In order to meet the goals of increased engine performance and reduced weight in advanced engine designs, new concepts for designing rotating engine components must be exploited. One solution which has been proposed is to use integrally bladed rotors, where the blades are integral with the disk rim. The elimination of mechanical blade attachments significantly reduces engine weight by reducing the size of the disk and its cascading effect on shaft size, bearing size, etc. Air leakage around blades is also eliminated, thereby increasing engine operating efficiency. Techniques for forming such types of rotors are described in, for example, commonly assigned U.S. Pat. Nos. 4,150,557 to Walker et al and 4,527,410 to MacNitt, Jr. et al, both incorporated by reference. The fabrication of integrally bladed rotors from superalloys such as IN100 requires that the forging process take place under superplastic conditions. One superplastic forging technique which has found widespread use in the industry is the Gatorizing.RTM. forging method (United Technologies Corporation, Hartford, CT), which is generally described in commonly assigned U.S. Pat. No. 3,519,503 to Moore and Athey, also incorporated by reference.
Superplastic forming is generally conducted at isothermal conditions; before the forming dies contact the component to be formed, the dies are preheated to a temperature which approximates the temperature to which the component is heated. When the dies and component are both at the desired temperature, the dies are brought into contact with the component and the forming operation takes place. Since the dies are made from materials which have excellent high temperature strength but poor high temperature oxidation resistance, an inert atmosphere is required to minimize (or prevent, if possible) oxidation or other thermal degradation of the dies. The atmosphere is contained within a sealed chamber, and the chamber completely surrounds the forming apparatus and the component.
The integrally bladed rotor technology developed to date has proven useful for fabricating rotors having a relatively small diameter, i.e., less than about 38 cm (about 15 in.). For the gas turbine engine industry to take full advantage of the benefits of integrally bladed rotors, the technology must be scaled up to the point where fabrication of rotors greater than about 45 cm (about 18 in.) in diameter can be made. The use of the prior art techniques does not appear to be economically efficient for making such large diameter rotors, since increases in rotor diameter requires larger and more complicated forming apparatus. Also, the inert atmosphere chambers for housing the forming apparatus become more complicated. Accordingly, what is needed is a method and apparatus for superplastically forming integrally bladed rotors in a more simple fashion.
The aforementioned patent to MacNitt, Jr. et al discloses that the manufacture of some integrally bladed rotors requires the use of multiple superplastic forming operations and dedicated apparatus for each operation. In particular, some rotor blades require multiple twisting operations to achieve the desired blade camber. Such multiple twist operations are costly, particularly in view of the capital equipment expenditure required to operate and maintain the specialized equipment. These problems also point to the need for more simple methods and apparatus for forming integrally bladed rotors.